An item of electronic equipment that dissipates more power than can easily be cooled with heat sinks alone generally uses fans to supplement natural convection. This works well enough, but as anyone who has labored in a room full of fan cooled equipment can attest, the noise from the fans themselves can be rather annoying. This is especially so in an office setting, where there arise issues of decorum, in addition to the more pragmatic issues of productivity reduction owing to distractions caused by noise.
A significant amount of fan noise appears to originate with the production of turbulent vortices of air at the tips of the fan blades as they rotate about the fan's axis. The tips pass sideways, as it were, through low pressure air located between the stationary venturi and the moving blade tips. As the blades rotate high pressure air spills over the tips of the blades and imparts an off axis spinning motion in the low pressure air (vortices) whose turbulent behavior results in the production of acoustic energy (noise).
It would be desirable if fan noise could be reduced without sacrificing the air flow the fan is intended to supply.